This would give us the ability to have "Cross-site ListPages" and could be quite useful. It was originally proposed by etnolinguisticahere (over 2 years ago) and more recently discussed later in the same thread. It would be most powerful if it accepted a comma separated list of site names so we could merge the results into one list. For example:

I was surprised there was never a formal wish for this. I suppose that could mean that there's little interest, but it seems like it should be fairly easy to implement. From a security standpoint, we could block others from listing our pages based on the setting used to block other cross-site includes.